1. Field of the Invention
The present invention relates to a method of processing image data for reproducing an image having good half-tone reproduction, sharpness, and details, through a pulse modulation of a laser beam, and an apparatus for executing the image processing method.
2. Discussion of the Related Art
In an image forming apparatus for reproducing a half-tone image through a pulse width modulation of a laser beam, such as a copying machine or a printer, as disclosed in Japanese Patent Unexamined Publication No. Sho. 53-19201, for example, a triangle wave signal is generated as a reference wave signal, and is compared with an analog image signal, and the laser beam is turned on and off in accordance with the result of the comparison.
In this method, to increase the sharpness of the reproduced image, it is necessary to increase the frequency of the reference wave signal. Further, to ameliorate a tone reproduction, it is necessary to decrease the frequency of the reference wave signal. To this end, a sharpness decision circuit is contained in an image processor section. A flag information signal outputted from the sharpness decision circuit is used for selecting one of a plurality of reference wave signals.
The triangle wave generating circuit is arranged so as to generate a triangle wave signal by integrating a reference clock signal. Therefore, it is difficult to keep the amplitude and the wave shape of these reference wave signals uniform.
To cope with this problem and, to improve the reliability, as described in Japanese Patent Unexamined Publication No. Hei. 5-276385, a sinusoidal wave signal is used in place of the triangle wave signal. However, an increase in the number of the reference wave signals leads to an increase in the number of required circuits, an increase in the cost to manufacture, and a defective image, such as interference.